1. Field of the Invention
The present invention relates to a heat-sensitive material which is suitable for the recording at a high density and high speed and which is superior in the thermal preservability.
2. Prior Art
In general, a heat-sensitive recording sheet is produced by applying a support, such as paper, synthetic paper, film, plastic, etc., a coating material which is prepared by individually grinding and dispersing a colorless chromogenic dyestuff and an organic color-developing agent, such as phenolic material, etc., into fine particles, mixing the resultant dispersion with each other and then adding thereto binder, filler, sensitizer, slipping agent and other auxiliaries. The coating material, when heated, undergoes instantaneously a chemical reaction which forms a color. These heat-sensitive recording materials have now been found in a wide range of application, including industrial measurement recorders, terminal printers of computer, facsimile equipment, automatic ticket vending machines, printer for barcode label, etc. In recent years, as the application of such recording is diversified and the performance of such recorders is enhanced, high qualities are required for heat-sensitive recording materials. For example, even with small heat energy in a high speed recording, the clear image with a high density is required.
However, the color formation with the small heat energy satisfying the above requirements sometimes brings about a serious problem. For example, the barcode label of a heat-sensitive recording material, which is widely used in supermarkets, etc., loses its function owing to the color-formation of the whole heat-sensitive layer of the label in the case the label is applied to a fried food such as fat-fried food. There is a contradiction that the above practical problem occurs easily with the increased color-forming ability in a small-heat energy. Under the above conditions, the color-formation with a small heat energy and the color-formation in the application of the label to a product under a high temperature are researched.
As a result, the following conclusion was obtained. The two color-formations are different from one another, i.e. the former corresponds to the so-called dynamic color-formation with an instantaneous heat-energy and the latter corresponds to a very specific case in the so-called static color-formation caused by a long contact with a high temperature-substance.
The heat in fried foods, etc. is transferred through a lap film and a support of thermal-sensitive label to the heat-sensitive layer, wherein the temperature of fried foods is usually at most 80.degree. C. in the lapse of excess-oil removal. However, the heat-sensitive layer is colored due to a long contact period with the heat.